[BUG][JAVA] Unable to Set Null Values for Nullable Fields in Generated Java Code

Description

When using the OpenAPI Generator to generate Java client code, nullable fields (defined with type: ["string", "null"]) cannot be set to null in the generated code, resulting in an incorrect request payload. Specifically, fields set to null are omitted from the JSON payload, which is problematic for PATCH requests where a missing field and a null field have distinct meanings.

openapi-generator version

7.13.0

OpenAPI declaration file content or url

{
  "openapi": "3.1.0",
  "info": {
    "title": "API Overview",
    "version": "1.0"
  },
  "servers": [
    {
      "url": "http://test.com",
      "description": "Test Server",
      "variables": {}
    }
  ],
  "paths": {
    "/users/{id}": {
      "parameters": [
        {
          "schema": {
            "type": "string"
          },
          "name": "id",
          "in": "path",
          "required": true,
          "description": "user ID"
        }
      ],
      "patch": {
        "summary": "Update a user by id",
        "operationId": "update-user-by-id",
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "type": "object",
                "properties": {
                  "id": {
                    "type": "string"
                  },
                  "name": {
                    "type": "string"
                  },
                  "phone": {
                    "type": ["string", "null"]
                  },
                  "email": {
                    "type": ["string", "null"]
                  }
                },
                "required": [
                  "id",
                  "email"
                ]
              }
            }
          },
          "description": ""
        },
        "responses": {
          "200": {
            "description": "default response",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "properties": {
                    "data": {
                      "type": "object",
                      "properties": {
                        "id": {
                          "type": "string"
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        },
        "tags": [],
        "parameters": []
      }
    }
  },
  "tags": [],
  "components": {
    "schemas": {}
  },
  "security": []
}

Generation Details

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i ./api.json -g java -o ./java_api_client

Steps to reproduce

public class App {
    public static void main(String[] args) {
        ApiClient defaultClient = new ApiClient();
        DefaultApi apiInstance = new DefaultApi(defaultClient);
        try {
            UpdateUserByIdRequest req = new UpdateUserByIdRequest();
            req.setEmail(null);
            req.setPhone(null);
            UpdateUserById200Response resp = apiInstance.updateUserById("123", req);
            // The request payload will be:
            // {}
        } catch (ApiException e) {
            throw new RuntimeException(e);
        }
    }
}

Expected Request Payload

{"email": null, "phone": null}

Actual Request Payload

{}

Start with the generated UpdateUserByIdRequest and ApiClient serialization behavior, using the supplied OpenAPI declaration and generation command. Trace how null-valued email and phone fields are handled before the request payload is sent. Done means the generated PATCH request preserves both fields as explicit JSON null values rather than omitting them.